The compact disc (CD) was invented in the 1980s to allow for an all-digital recording of audio signals. The optical pick-up unit (OPU) for audio-CD and/or CD-ROM uses a near-infrared (NIR) 780 nm semiconductor laser to read-out the encoded digital information. The numerical aperture (NA) of the objective lens is about 0.45, allowing a pit (one unit of encoding on disc) measuring about 100 nm deep, 500 nm wide and 850 nm to 3500 nm long depending on the radial distance from the disc center.
The first commercial digital versatile disc (DVD) appeared in the 1990s, with crucial optical design changes to allow for a physical recording density increase of about 3.5 times CDs. The gain in physical density was made possible by employing a shorter wavelength semiconductor (SC) laser (e.g., 650 nm, 660 nm red band, etc, compared to 780 nm near-IR band (NIR) in audio-CD) and a larger NA lens (e.g., 0.6 NA requiring a 0.6 mm thick DVD disc). A backward compatible DVD/CD optical pick-up unit employs two laser sources, either packaged as a single component or discretely, that have their read beams coupled by polarization beam combiners (PBCs) and/or dichroic beam combiners (DBCs).
Successors to the DVD media format range from Blu-ray Disc (BD) to high density HD-DVD. In these systems, the read/write SC laser wavelength is further decreased to about 405˜410 nm blue-violet band and the NA is increased to about 0.85. In BD or HD-DVD backward compatible DVD/CD systems, a third wavelength laser (e.g., co-packaged or discrete with respect to the first two lasers) is required to support all three disc media formats.
Referring to FIG. 1, there is shown one example of a prior art 3-wavelength HD-DVD/DVD/CD optical pick-up unit (OPU). The OPU 10 includes an array of semiconductor laser sources 20 (i.e., shown as three discrete laser diodes (LD) including a first LD 21 at λ=780 nm, a second LD 22 at λ=660 nm, and a third LD 23 at λ=405 nm), the output of which are spatially multiplexed by an array of polarization beam combiner (PBC) cubes 30, is collimated by a lens system 35 and is folded by a leaky mirror 40 before being imaged (focused) onto a single “pit” area on the rotating disc media 42 via an objective lens 36. The leaky mirror 40 allows for a small fraction (e.g., 5%) of the incident beam energy to be tapped off and focused onto a monitor photodiode (PD) 46 via another lens 37.
The output from the array of LD sources 20 is substantially linearly polarized (e.g., ‘S’ polarized with respect to the PBC hypotenuse surface). Prior to reaching the array of PBC cubes 30, these linearly polarized beams are transmitted through an array of low-specification polarizers 25, which protect the LD sources from unwanted feedback (e.g., “P” polarized light). In general, each polarizer 25 is a polarizing beamsplitter used to reflect light in one polarization state and to transmit light in the orthogonal polarization state. Conventionally, the protection filters 25 are simple dichroic absorptive polarizers with a 10:1 polarization extinction ratio.
The main ray from each of the LD sources 20 is directed along the common path 49 towards the disc media 42. Prior to reaching the quarter-waveplate (QWP) 41, the light is substantially linearly polarized. After passing through the QWP 41, the linearly polarized (LP) light is transformed into circularly polarized (CP) light. The handedness of the CP light is dependent on the optic axis orientation of the QWP (for a given S- or P-polarized input). In the example shown, with ‘S’ polarization input to the QWP, if the slow-axis of the QWP is aligned at 45° counter clockwise (CCW), with respect to the p-plane of the PBC, a left-handed circularly (LHC) polarized results at the exit of the QWP (LHC, having a Jones vector [1 j]T/√2 and with the assumption of intuitive RH-XYZ coordinate system while looking at the beam coming to the observer).
In a pre-recorded CD and/or DVD disc, where there is a physical indentation of a recorded pit, the optical path length difference between a pit and its surrounding “land”, at ⅙ to ¼ wave, provides at least partial destructive interference and reduces the light detected by the main photodiode 45 positioned at the second port of the PBC cube array 30. On the other hand, the absence of a pit causes the change of the CP handedness, at substantially the same light power in its return towards the PBC cube array 30. The light has effectively been transformed by the QWP in double-passing to convert the initially S-polarized light to P-polarized light on its return to the PBC array 30.
In the configuration illustrated in FIG. 1, where the reflected-port of the PBC array 30 is used to transmit the multiplexed LD output along the common light path 49, the absence of a pit signals polarization conversion and a high PD output is detected (on-state). The original CD-DA Red Book specification calls for greater than 70% reflectivity. This light is substantially P-polarized. The presence of a pit causes at least partial destructive interference between the light reflected from the pit and the surrounding land. The original CD-DA Red Book specification calls for no more than 28% reflectivity. This off-state light is not necessarily entirely P-polarized, as it is the result of diffraction (e.g., although it traverses the QWP twice). Some of the off-state S-polarized light and P-polarized light appears at the reflected port of the PBC array 30 and is steered towards the LD array 20. The array of protection filters 25 is provided to block this unwanted destructive light interference steered towards the LD array 20 (e.g., P-polarized light).
As discussed above, the protection filters 25 conventionally used are simple dichroic absorptive polarizers. Dichroic polarizers, such as the Polaroid-type dye sheet polarizer developed by E. H. Land in 1929, derive their diattenuation by selective anisotropic absorption of incident light. Due to this absorption, these sheet polarizers are not typically suitable for high power applications. In the backward compatible BD/HD-DVD 3-wavelength OPU system illustrated in FIG. 1, the blue-violet light at approximately 405 nm can have a power level between 100 and 500 mW when used as a write beam. In addition to this high power, the relatively short wavelength can potentially cause photo-induced damage to the dye sheet polarizers.
Unfortunately, the selection of a suitable replacement has not been obvious. While inorganic non-absorptive crystal polarizers in cemented prism form, such as Glan-Thomson or Glan-Taylor crystal polarizers, are more durable, they are also more expensive. Similarly, MacNeille-type polarizing beamsplitters, which rely on a stack of quarter-wave layers of isotropic material oriented at Brewer's angle for one of the polarization states, and wiregrid polarizers, such as those offered by Moxtek, are relatively expensive.
One reflective polarizer that is used often in various applications is the multi-layer polymer film offered by 3M. Like wiregrid polarizers, this multi-layer polymer film is a Cartesian polarizer. In a Cartesian polarizer, the polarization of the reflected and transmitted beams of light is referenced to invariant, generally orthogonal principal axes of the Cartesian polarizer. In general, the polarization of the reflected and transmitted beams of light is substantially independent of the incident angle. Accordingly, Cartesian polarizers are often associated with good angular acceptance.
In the 3M Cartesian polarizer, the multi-layer optical film includes a stack of alternating isotropic and birefringent polymer materials. The birefringent polymer materials are typically configured substantially as A-plate retarders, while the isotropic materials are typically selected to have substantially the same index of refraction as one index of refraction of the birefringent polymer materials. Alternatively, the multi-layer optical film includes a stack of alternating birefringent polymer materials configured as A-plate retarders and C-plate retarders, respectively. Notably, the stack of alternating isotropic and/or birefringent polymer materials uses what is often referred to as Giant-Birefringence Optics (GBO) effect. Unfortunately, the alternating series of isotropic and/or birefringent layers is typically created by polymer extrusion and involves mechanically stretching the polymers to induce birefringence. As a result, the polarizing film is susceptible to photo- and thermal-induced degradation and is not ideal for use in a backward compatible BD/HD-DVD 3-wavelength OPU system.
Another example of a reflective Cartesian polarizer is based on a cholesteric liquid crystal (ChLC). In a ChLC polarizer, the ChLC molecules are ordered to form a helical structure having a pitch p. The ChLC polarizer transmits right- or left-handed circularly polarized light at a wavelength corresponding to the optical length of the pitch of the ChLC. The light that is not transmitted is reflected and is circularly polarized in the opposite helicity. In general, the ChLC material will be sandwiched between two quarter-wave retarders to convert the transmitted circularly polarized light into linearly polarized light.
ChLC reflective polarizers based on stable liquid crystal polymer (LCP) have also been proposed. LCPs are a type of polymer in which liquid crystal monomers are incorporated into the macromolecular structure along the main-chain (backbone) or as side chain units. LCPs are typically aligned via mechanically brushing or photo-alignment. In general, photo-alignment techniques typically involve first applying either a linear photo-polymerizable (LPP) film, or an azo-based dye, in a polymer alignment layer. If a LPP film is used, the LPP media is deposited on a substrate and then cured at an elevated temperature. The cured film is then subjected to polarized UV light for alignment. The alignment is then fixed with a cross-linking step. Once this photosensitive layer is aligned, the LCP layer is applied (e.g., spin-coated) over the LPP layer. As is well known in the art, the LCP layer typically includes cross-linkable liquid crystalline monomers, oligomers, or polymer precursors having cross-linkable groups. Once the LCP layer is aligned with the orientation of the LPP film, the LCP layer is then cross-linked by exposure to unpolarized UV light. Photochemical cross-linking of the alignment layer polymer and the subsequent cross-linking of the LCP molecules is expected to improve the reliability of the polarizer under high power illumination and short wavelength laser exposure.
ChLC reflective polarizers formed from stable LCP media have been used in various liquid crystal display (LCD) projection systems. In these systems, the polarizers are typically fabricated as broadband reflective polarizers, and thus are expensive relative to dichroic absorptive polarizers. In particular, the ChLC layer is typically fabricated with a highly chirped helical pitch, whereas the two more quarter-wave retarders are designed to be achromatic, so that the reflective polarizer is targeted for all of the Red, Green and Blue (RGB) wavelength channels (e.g., center wavelength ±10%). If fact, to the best knowledge of the instant inventors, ChLC reflective polarizers have not been used at the very short wavelength (e.g., 405 nm) for BD or HD-DVD channel in backward compatible CD/DVD OPU systems.